Giant aluminum company Alcoa Inc., reacting quickly to a three-way, intercontinental merger aimed at challenging its dominance, disclosed an unsolicited takeover bid for rival Reynolds Metals Co., offering $4.2 billion, or $66.44 a share, in cash and stock, plus the assumption of $1.5 billion in debt.

Alcoa's bid, which if successful is likely to draw intense regulatory scrutiny, is all the more brassy because it homed in on Reynolds's vulnerable position. The best alternate merger partners for Reynolds -- Canada's Alcan Aluminium...